This invention relates to a flow-velocity sensor probe used in flow-velocity measurement such as measurement of cardiac output.
In measurement of cardiac output which is essential in managing critically ill patients with cardiac failure, conventionally use is made of methods that rely upon ultrasound, dye dilution and radioisotopes, etc. Owing to its simplicity and accuracy, wide use is made of a thermodilution method based upon the right-heart catheter method, in which a catheter is held in the pulmonary artery.
However, the information obtained in the thermodilution method is discontinuous. In addition, when cardiac output is measured, an infusion fluid must be injected each time a measurement is taken. Owing to such problems as the complexity of surgery, infection which accompanies the repeated injection of a cold saline solution, a drop in body temperature and increased load on the heart, the number of times measurements can be taken is limited, especially in case of a seriously ill patient whose cardiac output needs to be ascertained.
One method of continuously measuring cardiac output very precisely is a method using the CCOM system (continuous cardiac output monitoring system) developed by the present inventors (U.S. Pat. No. 4,685,470). This monitoring system includes a catheter probe and a monitoring unit. By continuously measuring the amount of heat loss, which is due to blood flow, using a thermistor (referred to as a CFT) self-heated by passage of a current therethrough, cardiac output is monitored continuously without the intermittent injection of a cold saline solution.
Cardiac output (CO) is the amount of blood expelled from the heart in a unit of time and usually is expressed by a value per minute. Ordinarily, if the heart or a major blood vessel is not short-circuited, the amount of blood expelled from the right heart and that expelled from the left heart are equal, and cardiac output CO (L/min) is obtained in accordance with the following equation from flow velocity (cm/sec) in the pulmonary artery and the cross-sectional area s (cm.sup.2) of the pulmonary artery: EQU CO=0.06.multidot.s.multidot.v (1)
The principle of continuous measurement of cardiac output will now be described.
A thermistor is used as an ordinary temperature sensor and operates on the basis of a change in resistance value in dependence upon a change in temperature.
Since a thermistor is also a resistor, the thermistor itself emits heat when a large amount of current is passed through it. Accordingly, when such a thermistor is placed in the bloodstream, the temperature of the thermistor becomes that at which equilibrium is established between the amount of heat produced by the electric current and the amount of heat carried away by the flow of blood. Since this equilibrium temperature Tt varies in dependence upon the flow velocity, the thermistor can be utilized as a flow-velocity sensor.
The relationship between equilibrium temperature Tt (C.degree.) and blood flow velocity (cm/sec) can be expressed by the following equation, which is based upon experimentation: EQU log Tt=A.multidot.log v+B (2)
where A and B are constants dependent upon the fluid and the characteristics of the thermistor, etc.
In order to measure cardiac output CO continuously, it is necessary to obtain a relation between the equilibrium temperature Tt and CO. Therefore, cancelling the flow velocity v from Eqs. (1) and (2) gives us the following equation: EQU log Tt=A.multidot.log CO+B-A.multidot.log (0.06.multidot.s)(3)
However, Eq. (3) includes an unknown parameter, namely the cross-sectional area s of the pulmonary artery, and cannot be used as is in measuring cardiac output. Accordingly, if cardiac output and equilibrium temperature are measured at least once and the measured values are substituted into Eq. (3) as calibration values CO.sub.CAL and Tt.sub.CAL, we have EQU log Tt.sub.CAL =A.multidot.log CO.sub.CAL +B-A.multidot.log (0.06.multidot.s) (4)
When the cross-sectional area s of the pulmonary artery is cancelled from Eqs. (3) and (4), we obtain EQU log (Tt/Tt.sub.CAL)=A.multidot.log (CO/CO.sub.CAL) (5)
Accordingly, cardiac output CO can be expressed by the following equation as a function of equilibrium temperature Tt: EQU CO=CO.sub.CAL .multidot.(Tt/Tt.sub.CAL).sup.1/A ( 6)
This makes it possible to measure cardiac output continuously using a self-heating thermistor.
A method of calculating cardiac output using the CCOM system will now be described.
In a CCOM system, the above-mentioned calibration is performed by the thermodilution method, and two thermistors are attached to a catheter probe. One of these thermistors is a self-heating CFT thermistor for measuring equilibrium temperature and a PAT thermistor for measuring blood temperature using the thermodilution method.
CFT thermistor temperature Tt.sub.R is dependent upon a change in blood flow velocity but is also dependent upon a change in blood temperature TB. Accordingly, a correction in Tt.sub.R which accompanies a change in blood temperature from the time of calibration is carried out in accordance with the following equation: EQU Tt=Tt.sub.R -K.multidot.(TB-TB.sub.CAL) (7)
where
Tt.sub.R : CFT thermister temperature at time of measurement PA1 K: blood temperature correction coefficient PA1 TB: blood temperature PA1 TB.sub.CAL : blood temperature at time of calibration
If the temperature correction of Eq. (7) is applied to Eq. (6), the following equation is obtained: EQU CO=CO.sub.CAL .multidot.{[Tt.sub.R -K.multidot.(TB-TB.sub.CAL)]/Tt.sub.CAL }.sup.1/A ( 8)
Thus, as set forth above, cardiac output CO can be calculated in accordance with Eq. (8) from the continuously measured CFT-thermistor temperature Tt.sub.R and blood temperature TB. However, when the full range (0-12 L/min) of cardiac output is calculated with the value of the constant A in Eq. (8) being a simple value, there is a decline in precision. Therefore, the range over which cardiac output is measured is divided into two parts. Specifically, cardiac output is calculated using arithmetic expressions for a case where the value of A when the cardiac output is in a high flow-rate region is AH and a case where the value of A when the cardiac output is in a low flow-rate region is AL. It should be noted that the constant A is an index of temperature with regard to flow velocity and shall be referred to as the "A value" hereinafter.
(1) Processing in a case where the calibration value CO.sub.CAL of cardiac output is greater than 2.75 L/min:
Initially, calibration of cardiac output is carried out by the thermal dilution method. Next, the CFT-thermistor temperature Tt.sub.2.75 when the cardiac output is 2.75 L/min is calculated from the calibration values (CO.sub.CAL and Tt.sub.CAL). That is, when Eq. (6) is transformed into an equation which obtains Tt and CO=2.75 L/min is substituted into the equation with the A value serving as AH, we have EQU Tt.sub.2.75 =Tt.sub.CAL .multidot.(2.75/CO.sub.CAL).sup.AH ( 9)
At the time of measurement, cardiac output is obtained in accordance with the following arithmetic expressions where the measurement range is divided into two parts: ##EQU1##
(2) Processing in a case where the calibration value CO.sub.CAL of cardiac output is less than 2.75 L/min:
As in the case of (1) above, the CFT-thermistor temperature Tt.sub.2.75 when the cardiac output is 2.75 L/min is calculated from the calibration values (CO.sub.CAL and Tt.sub.CAL). That is, the A value is adopted as AL as the following is obtained from Eq. (6): EQU Tt.sub.2.75 =Tt.sub.CAL .multidot.(2.75/CO.sub.CAL).sup.AL ( 12)
At the time of measurement, cardiac output is obtained in accordance with the following arithmetic expressions where the measurement range is divided into two parts: ##EQU2##
FIG. 1 illustrates the structure of a conventional flow-velocity sensor probe (catheter probe). The probe includes a catheter tube 1 and a balloon inflating line 9, a pressure measuring line 10, an infusion fluid injecting line 11 and a thermistor connecting line 12, all of which are connected to the catheter tube 1 via a manifold 6.
The structure of the catheter tube 1 is such that a pressure measuring aperture 4, a CFT thermistor 2 and a PAT thermistor 3 are disposed at the tip of the catheter tube. The CFT thermistor 2 and PAT thermistor 3 are electrically connected to a CFT connector 7 and a PAT connector 8, respectively.
FIG. 2 illustrates the structure of the CFT thermistor mount in the conventional flow-velocity sensor probe.
As shown in FIG. 2, the CFT thermistor 2 is dipped in a waterproof epoxy resin 34 in order to assure a waterproof condition and is then inserted into a tube 31 made of polyimide. The CFT thermistor 2 inserted into the tube 31 is fitted into a side aperture 29 in the catheter tube 1, and the thermistor 2 is then bonded into place by an epoxy bonding agent 36 in order to fix the thermistor to the catheter tube 1.
Thermistor leads 32 are passed through the interior of the catheter tube 1 and are electrically connected to the CFT connector 7 of the thermistor connecting line 12.
A number of problems are encountered in the prior art. Specifically, in the conventional CCOM system described above, the change in the temperature of the CFT thermistor regarding blood flow is small and the sensitivity needed in order to measure cardiac output is unsatisfactory. More specifically, in the CFT thermistor mount of the flow-velocity sensor probe (catheter probe), the structure is such that a resin having poor thermal conductivity located between the CFT thermistor and the outside (blood) blocks the efficient transfer of heat, which is emitted by the CFT thermistor, to the outside, and therefore cooling by the blood cannot be carried out sufficiently.
In addition, the conventional flow-velocity probe (catheter probe) allows escape of heat along the thermistor leads, and this has an affect upon the temperature of the PAT thermistor.